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Advanced Organic Chemistry: 1H NMR spectrum of 1-chloropropane

The H-1 hydrogen-1 (proton) NMR spectrum of 1-chloropropane

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry courses involving molecular spectroscopy analysing H-1 NMR spectra of 1-chloropropane

C3H7Cl CH3CH2CH2Cl low and high resolution 1H proton nmr spectrum of 1-chloropropane analysis interpretation of chemical shifts ppm spin spin line splitting diagram H1 H-1 nmr for n-propyl chloride explaining spin-spin coupling for line splitting doc brown's advanced organic chemistry revision notes

TMS is the acronym for tetramethylsilane, formula Si(CH3)4, whose protons are arbitrarily given a chemical shift of 0.0 ppm. This is the 'standard' in 1H NMR spectroscopy and all other proton resonances, called chemical shifts, are measured with respect to the TMS, and depend on the individual (electronic) chemical environment of the hydrogen atoms in an organic molecule - 1-chloropropane here.

The chemical shifts quoted in ppm on the diagram of the H-1 NMR spectrum of 1-chloropropane represent the peaks of the intensity of the chemical shifts of (which are often groups of split lines at high resolution) AND the relative integrated areas under the peaks gives you the ratio of protons in the different chemical environments of the 1-chloropropane molecule.

1-bromopropane  C3H7Cl          

Interpreting the H-1 NMR spectrum of 1-chloropropane

In terms of spin-spin coupling from the possible proton magnetic orientations, for 1-chloropropane I have only considered the interactions of non-equivalent protons on adjacent carbon atoms e.g. -CH2-CH3, or -CH2-CH2 protons.

For relatively simple molecules, the low resolution H-1 NMR spectrum of 1-chloropropane is a good starting point (low resolution diagram above).

The hydrogen atoms (protons) of 1-chloropropane occupy 3 different chemical environments so that the low resolution NMR spectra should show 3 principal peaks of different H-1 NMR chemical shifts (diagram above for 1-chloropropane).

CH3CH2CH2Cl

Note the proton ratio 3:2:2 of the 3 colours of the protons in the 3 chemically different environments

Chemical shifts (a) to (c) on the H-1 NMR spectrum diagram for 1-chloropropane.

Although there are 7 hydrogen atoms in the molecule, there are only 3 possible different chemical environments for the hydrogen atoms in 1-chloropropane molecule.

The integrated signal proton ratio 3:2:2 observed in the low/high resolution H-1 NMR spectrum, corresponds with the structural formula of 1-chloropropane.

The high resolution 1H NMR spectrum of 1-chloropropane

All low and high resolution spectra of 1-chloropropane show 3 groups of proton resonances and in the 3:2:2 ratio expected from the formula of 1-chloropropane.

The ppm quoted on the diagram represent the peak of resonance intensity for a particular proton group in the molecule of 1-chloropropane - since the peak' is at the apex of a band of H-1 NMR resonances due to spin - spin coupling field splitting effects - see high resolution notes on 1-chloropropane below.

So, using the chemical shifts and applying the n+1 rule to 1-chloropropane and make some predictions using some colour coding! (In problem solving you work the other way round!)

(a) 1H Chemical shift 0.85 ppm, CH3 protons: CH3CH2CH2Cl

The methyl resonance is split by the adjacent CH2 protons into a 1:2:1 triplet (n+2 = 3).

Evidence for the presence of a CH2 group in the molecule of 1-chloropropane

(b) 1H Chemical shift 1.61 ppm, CH2 protons: CH3CH2CH2Cl

Th CH2 resonance is split by the adjacent CH2 and CH3 protons into a 1:5:10:10:5:1 sextet (n+5 = 6).

Evidence for the presence of a CH3CH2 group in the molecule of 1-chloropropane

(c) 1H Chemical shift 3.30, CH2 protons : CH3CH2CH2Cl

The methyl resonance is split by the adjacent CH2 protons into a 1:2:1 triplet (n+2 = 3).

Evidence for the presence of a 2nd CH2 group in the molecule of 1-chloropropane

Evidence for the presence of a ? group in the molecule of 1-chloropropane

Note the decreasing effect on the 1H chemical shift as the proton is further from the more electronegative oxygen and nitrogen bromine chlorine atoms 1-chloropropane.


Number of directly adjacent protons 1H causing splitting Splitting pattern produced from the n+1 rule on spin-spin coupling and the theoretical ratio of line intensities
0 means no splitting             1            
1 creates a doublet           1   1          
2 creates a triplet         1   2   1        
3 creates a quartet       1   3   3   1      
4 creates a quintet     1   4   6   4   1    
5 creates a sextet   1   5   10   10   5   1  
6 creates a septet 1   6   15   20   15   6   1

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